Do I always need planning permission to turn a garage into a flat?

In most cases, yes. Ordinary conversions fall under permitted development because the work is internal and the building is not enlarged, but creating a separate dwelling is a different proposition. The Planning Portal advises that permission may be required regardless of how much physical building work is involved. Permission is also needed if your permitted development rights have been removed, if the property is listed or in a conservation area, or if a planning condition requires the garage to stay available for parking. Check with your local planning authority before you commission drawings, since their view on parking and amenity space will shape the design.